A switching power supply provides a high-efficiency regulated output voltage for a variety of power-management functions that are often required in modern electronic systems. These systems have either a battery or an unregulated power supply as their input power sources, and may have multiple power output voltages that require power conversion, power management functions, and tight regulation.
The function of a switching power supply is to convert power from an input power source to output power for an electronic system. This conversion should be at high efficiency for the various operational modes of the system. For example, when a notebook computer or cellular phone system is operated in a low-power mode, it is desired to extend battery life. On the other hand, a well-regulated voltage supply is desired when the notebook or phone is operated in a high-power mode. Furthermore, the switching power supply is required to respond with little or no regulation error when the notebook or phone changes from a low- to a high-power mode.
Traditionally, a switching power supply provides a regulated output voltage by controlling a supply current through a power switching stage. The switched current is connected to a filter stage, which reduces the resulting ripple voltage. A feedback stage measures the regulated output voltage and, in response to the measured value of the regulated output voltage, generates a control signal that controls the power switching stage.
To regulate the output voltage of a pulse-width-modulated (PWM) switching power supply, the control signal activates the power switching stage when the regulated output voltage drops below a threshold voltage (for a positive regulated output voltage) and then deactivates the switching power stage when the regulated output voltage increases above the threshold voltage. The pulse width of the control signal determines the period of time that the power switching stage provides current to the filter stage, and hence, the supply regulates the output voltage by controlling the widths of the control-signal pulses. Therefore, the pulse width of the control signal is proportional to the current being drawn by the load, and the frequency of the control signal is typically constant.
To regulate the output voltage of a constant-on-time switching power supply, the control signal activates the power switching stage for a predetermined constant time when the regulated output voltage drops below a threshold voltage (for a positive voltage). Therefore, the pulse width of the control signal is constant, and the frequency of the control signal is proportional to the current drawn by the load. Ideally, under steady-state conditions, the supply operates at a constant switching frequency without a synchronizing signal. But in noisy environments, noise may be superimposed on the output voltage or threshold voltage, and this noise may cause jitter in the phase, and thus the frequency, of the control signal. Unfortunately, this jitter may cause the power supply to irradiate electromagnetic interference that can adversely affect the electronic system that incorporates the power supply or nearby electronic systems.